In the production of particle board, this term being used generically for fiberboard, chipboard, sawdust board and boards made from mixtures of cellulosic fibers and chips, it is a common practice to form a mat of the particulate material upon a receiving surface, the mat consisting of the particles to be bonded together by intrinsic binding agents or a mixture of the particles with a binder. Intrinsic binding agents may be the abietic resins normally present in wood particles while the additional synthetic-resin binding agents can be of the thermally hardenable or thermosetting type. Suitable binders for this purpose are the phenol-formaldehyde, urea, resorcinol or melamine resins.
The mat, of a particular thickness to provide the necessary quantity of material per unit of surface area, is compacted, generally with heating, in a platen or other press to activate the resin and cause it to bond the particles together into a coherent structure. Depending upon the amount of binder present, the degree and intensity of compression and the nature of the heating cycle, such particle boards can be of extremely low porosity and high density for use as structure materials, can be of moderate porosity and strength for use as a facing material, or can be of high-porosity and relatively low strength for thermal and acoustical insulation purposes.
It is known to form such mats by dispensing the particulate matter upon the receiving surface, the latter preferably being a conveyor or the like which is displaced past a dispensing head.
In general, however, the formation of the mat is not so simple. Firstly, it is necessary to have complete control of the amount of the particulate material which is deposited per unit area of the receiving surface so that the particle board, upon compression of the mat, will have the desired density and uniformity. Secondly, it is important to dispense the particulate material in such spread, distributed or nonagglomerated form as to also insure uniformity in the product. Thirdly, the system must be capable of depositing materials of different particle sizes and character, e.g. fibers, chips and dust, depending upon the nature of the product which is desired.
Thus the art of forming the mat or layer of particulate matter upon a receiving surface has become quite sophisticated.
It is known, for example, to provide a layer-forming apparatus for depositing particulate matter as particle-board mats upon a receiving surface, especially for the production of particle board by hot-pressing, with the following main components:
a. A particulate-material hopper with a conveyor-belt floor, a device for feeding particulate matter into the hopper, and a discharge roller grate forming an end wall of the hopper on the side toward which the particulate material is moved by the conveyor, the roller grate being inclined downwardly in the direction of feed of the particulate material. The particulate material is thereby predominantly dispensed through the roller grate and from the leading end of the conveyor floor. This arrangement has the advantage that the dispensing process permits the product to cascade downwardly through the interstices of the roller grate with a minimum of compaction and agglomeration and with a certain dispersal of the particles.
b. A metering device with a metering conveyor disposed below the hopper and provided with means for controlling the quantity of particulate material which is carried by the metering conveyor. The latter means can include a volume-metering roller, a weight-metering roller or a combination of the two, the latter regulating the rate of which the particulate matter is advanced toward the receiving surface. At the discharge end of the metering conveyor, a cast-off roller can be provided to propel the particulate matter from the conveyor.
c. A spreading head receiving the particulate matter from the cast-off roller and spreading it upon the receiving surface. The latter can be disposed beneath the spreading head and can be used to carry the mat or final layer to the press.
Such an apparatus is described, for example, in German Offenlegungsschrift DT OS No. 22 23 779. It has been found to be extremely effective in the preparation of mats for particle board.
Frequently, in the fabrication of particle board, it is desirable to have a uniform surface zone on one or both of the faces thereof. It is known, for example, to apply facing laminates to a particle-board core to achieve esthetic results or to separately apply a laminate of low permeability to prevent moisture penetration or the like. Such steps are labor-consuming and expensive and are not always effective. It thus has long been sought to be able to provide a surface zone of fine particles on one or both faces of a particle board using otherwise conventional mat-forming apparatus and hence at minimum cost and with a maximum effectiveness.